Performance Review of Strain-Hardening Cementitious Composites in Structural Applications

Strain-hardening cementitious composites (SHCC) are an attractive construction material with obvious advantages of large strain capacity and high strength, as well as excellent workability and easy processing using conventional equipment. Moreover, SHCC can be designed with varied mix proportions in order to satisfy various requirements and expectations to overcome the shortages of existing construction materials. However, the behavior of SHCC in the structural application is varied from that of SHCC material, which is reviewed and presented in this paper, focusing on the flexural and shear behavior of the SHCC member and the SHCC layer used for strengthening reinforced concrete (RC). The reviewed results demonstrate that both the zero-span tensile behavior of the stress concentration and the uniaxial tensile behavior of the bending effect can influence the crack propagation patterns of multiple fine cracks in the SHCC strengthening layer, in which the crack distribution within the SHCC layer is limited near the existing crack in the RC substrate member in the zero-span tensile behavior. Moreover, the crack propagation patterns of the SHCC strengthening layer are changed with varied layer thicknesses, and the SHCC strengthening layer, even with a small thickness, can significantly increase the shear load carrying capacity of the shear strengthened RC member. This work provides the foundations for promoting SHCC material in the structural application of repairing or retrofitting concrete structures.


Introduction
Concrete structures are easily damaged due to the weakness of concrete in resisting tensile forces with brittleness, and many different construction materials have been developed for repairing or strengthening aged and damaged concrete structures. However, the typical existing construction materials have obvious shortages in strengthening concrete structures, in which the performance of the fiber-reinforced polymer (FRP) strengthening layer is strictly related to the bond and compatibility between FRP and concrete [1][2][3]; epoxy-bonded concrete is easily delaminated due to epoxy expansion caused by solar radiation [4], fiber-reinforced concrete (FRC) is still quasi-brittle during the repairing works [5][6][7], and polymer cement mortars (PCM) have limitations in terms of high costs [8].
In recent decades, strain-hardening cementitious composites (SHCC) have been developed with the obvious advantages of a large strain capacity, high strength, self-healing capacity, excellent workability, and easy processing using conventional equipment [9][10][11]. In particular, SHCC can be designed with varied mix proportions, which is expected to overcome the shortages of existing construction materials and satisfy the various requirement in engineering construction. In view of previous studies, the behavior of SHCC in structural applications has varied from that of SHCC materials, whereas the property of SHCC materials is closely connected with the behavior of SHCC in structural applications [12][13][14][15]. Table 1 demonstrates the findings of the reviewed sources on the property of As mentioned above, the behavior of SHCC in structural applications is quite different from that of SHCC material, since the short fibers distributed in large-sized structures using SHCC are significantly varied from those distributed in small specimens for obtaining the material properties of SHCC [22,23]. In this paper, the structural performance of SHCC is systematically reviewed, focusing on the flexural and shear behavior of the SHCC member and the SHCC layer used for strengthening the RC member.

Material Property of SHCC Material
As mentioned above, SHCC can be designed with varied mix proportions, the main ingredients being water, cement, silica fume, fine sand, and fiber. Figure 1 shows the stress-strain curves and cracking pattern of the dumbbell-shaped SHCC specimens, which are obtained from the uniaxial tensile test. The dumbbell-shaped specimens have a 13 mm × 30 mm cross-section, and the strain is defined as the elongation rate with a 100 mm measured length [24]. In the mix proportion of this SHCC material, the water-tobinder ratio is 22%, and there is 338.5 kg of water, 1267.9 kg of cement, 230.8 kg of silica fume, 153.9 kg of fine sand, 14.6 kg of PE fiber, 15.4 kg of super plasticizer, and 0.06 kg of air reducing agent in per cubic meter of the SHCC material. Moreover, this SHCC material is also employed in the following experiment of the SHCC member and the SHCC layer used for strengthening the RC member.

Netherlands
To evaluate mechanical behavior of printed SHCC Printing technique guarantees an enhanced bond between the printed SHCC layers As mentioned above, the behavior of SHCC in structural applications is quite different from that of SHCC material, since the short fibers distributed in large-sized structures using SHCC are significantly varied from those distributed in small specimens for obtaining the material properties of SHCC [22,23]. In this paper, the structural performance of SHCC is systematically reviewed, focusing on the flexural and shear behavior of the SHCC member and the SHCC layer used for strengthening the RC member.

Material Property of SHCC Material
As mentioned above, SHCC can be designed with varied mix proportions, the main ingredients being water, cement, silica fume, fine sand, and fiber. Figure 1 shows the stress-strain curves and cracking pattern of the dumbbell-shaped SHCC specimens, which are obtained from the uniaxial tensile test. The dumbbell-shaped specimens have a 13 mm × 30 mm cross-section, and the strain is defined as the elongation rate with a 100 mm measured length [24]. In the mix proportion of this SHCC material, the water-to-binder ratio is 22%, and there is 338.5 kg of water, 1267.9 kg of cement, 230.8 kg of silica fume, 153.9 kg of fine sand, 14.6 kg of PE fiber, 15.4 kg of super plasticizer, and 0.06 kg of air reducing agent in per cubic meter of the SHCC material. Moreover, this SHCC material is also employed in the following experiment of the SHCC member and the SHCC layer used for strengthening the RC member. As demonstrated in Figure 1, the initial tension strength of the SHCC material is 4.2 MPa at point A, and the ultimate tensile strengths with a lower limit and an upper limit, is 7 MPa and 8 MPa at points B1 and B2, respectively, corresponding to the hardening strains of 1.5% and 2.2%. In all of the specimens, the initial cracks firstly appear at the initial tension strength at point A, and then the multiple fine cracks with significant strain hardening behavior occur and propagate until the ultimate tension strength at points B1 or B2. Thereafter, the stress decreases due to the localization of the multi-fine cracks. As demonstrated in Figure 1, the initial tension strength of the SHCC material is 4.2 MPa at point A, and the ultimate tensile strengths with a lower limit and an upper limit, is 7 MPa and 8 MPa at points B1 and B2, respectively, corresponding to the hardening strains of 1.5% and 2.2%. In all of the specimens, the initial cracks firstly appear at the initial tension strength at point A, and then the multiple fine cracks with significant strain hardening behavior occur and propagate until the ultimate tension strength at points B1 or B2. Thereafter, the stress decreases due to the localization of the multi-fine cracks. Figure 2 shows the load-displacement curve and crack propagation pattern of the SHCC member in flexural failure, in which the geometry of the experimental specimen is 1800 mm long with a cross-section of 150 mm × 200 mm. Moreover, there are no stirrups and longitudinal reinforcements arranged in the SHCC member. The specimen is loaded under a four-point bending setup, the displacements and load are measured using displacement transducers and a load cell, respectively, which are marked as A and B, respectively. The range of the load cell is 300 kN, with 0.1 kN sensitivity, and that of the displacement transducer is 25 mm with 0.002 mm sensitivity. During the experiment, the displacement transducer is required to be adjusted when the displacement is larger than 25 mm.  Figure 2 shows the load-displacement curve and crack propagation pattern of the SHCC member in flexural failure, in which the geometry of the experimental specimen is 1800 mm long with a cross-section of 150 mm × 200 mm. Moreover, there are no stirrups and longitudinal reinforcements arranged in the SHCC member. The specimen is loaded under a four-point bending setup, the displacements and load are measured using displacement transducers and a load cell, respectively, which are marked as A and B, respectively. The range of the load cell is 300 kN, with 0.1 kN sensitivity, and that of the displacement transducer is 25 mm with 0.002 mm sensitivity. During the experiment, the displacement transducer is required to be adjusted when the displacement is larger than 25 mm. The crack propagation pattern of the SHCC member corresponding to the points 1 to 4 in the load-displacement curve is also shown in Figure 2. It can be clearly seen that the flexural multiple fine cracks appear in the SHCC member first (point 1), and then those are significantly increased with the increasing load carrying capacity (point 2), and finally, fails due to the localization of some flexural multiple fine cracks thereafter (points 3 and 4). This means the SHCC member experiences the multi-cracking processes, whereas the multiple fine cracks distributed in the flexural-failed SHCC member are reduced from those of the SHCC material in uniaxial tension. Figure 3 shows the shear stress-displacement curve and crack propagation pattern of the SHCC members in diagonal shear failure [24], in which cases 2-100 and 3-100 have the ratios of shear span length to effective depth (a/d) equaling 2 and 3, respectively. In both cases, 2-100 and 3-100 have a 100 mm width and a 150 mm effective depth, and the shear span lengths of the cases 2-100 and 3-100 are 300 mm and 450 mm, respectively. Moreover, the 25-mm diameter high tensile strength deformed bar with a 200 GPa Young's modulus and a 1050 MPa yield strength is arranged in the longitudinal direction. In the experiment, the displacements at the loading points and supporting points, as well as the middle point, are measured by displacement transducers, and the load is measured by a load cell, which are marked as A and B, respectively. The loading test is terminated when a sudden drop is observed during the loading process. The crack propagation pattern of the SHCC member corresponding to the points 1 to 4 in the load-displacement curve is also shown in Figure 2. It can be clearly seen that the flexural multiple fine cracks appear in the SHCC member first (point 1), and then those are significantly increased with the increasing load carrying capacity (point 2), and finally, fails due to the localization of some flexural multiple fine cracks thereafter (points 3 and 4). This means the SHCC member experiences the multi-cracking processes, whereas the multiple fine cracks distributed in the flexural-failed SHCC member are reduced from those of the SHCC material in uniaxial tension. Figure 3 shows the shear stress-displacement curve and crack propagation pattern of the SHCC members in diagonal shear failure [24], in which cases 2-100 and 3-100 have the ratios of shear span length to effective depth (a/d) equaling 2 and 3, respectively. In both cases, 2-100 and 3-100 have a 100 mm width and a 150 mm effective depth, and the shear span lengths of the cases 2-100 and 3-100 are 300 mm and 450 mm, respectively. Moreover, the 25-mm diameter high tensile strength deformed bar with a 200 GPa Young's modulus and a 1050 MPa yield strength is arranged in the longitudinal direction. In the experiment, the displacements at the loading points and supporting points, as well as the middle point, are measured by displacement transducers, and the load is measured by a load cell, which are marked as A and B, respectively. The loading test is terminated when a sudden drop is observed during the loading process. The shear stress is defined by the shear force divided by the effective section area of the effective height multiplied by width, and the maximum shear stresses of cases 2-100 and 3-100 are 9.8 MPa and 7.0 MPa, respectively, which means the specimen with a/d = 2 has a larger load carrying capacity than that of the specimens with a/d = 3. The shear stress-displacement curve also shows that the linear curve is near the maximum shear strength and the sudden shear failure occurs thereafter. This behavior may be due to the smooth crack surface of the SHCC member.

Shear Stress-Displacement Relationship and Crack Propagation Pattern
The crack propagation pattern of the tested specimens corresponding to the points 1 to 5 in the shear stress-displacement curve is also demonstrated in Figure 3. In the case 2-100, the flexural multi-fine cracks first occur at point 1 (lower load level with a 2 MPa shear stress), and some multiple fine cracks occur in the middle of the shear span independent of the aforementioned flexural cracks at point 2 (load level with 4 MPa shear stress). At points 3 and 4 (load level with 7.3 MPa and 9.3 MPa shear stresses), the multiple fine cracks are gradually increased with the increasing load level, in which the multiple fine cracks in the shear direction play a dominant role in the SHCC member in shear failure. Moreover, it is obvious that the localization behavior occurs among the multiple fine cracks at point 5 (peak load with a 9.8 MPa shear stress), and the shear load suddenly drops thereafter, in which the shear angle of the multiple fine cracks between the diagonal shear direction and axial direction is about 33.5°.
In the crack propagation pattern of case 3-100, it can be easily seen that this specimen shows similar crack propagation behavior to that of case 2-100. The flexural multi-fine cracks first occur at point 1 (a lower load level with a 1.7 MPa shear stress), then the multiple fine cracks occur in the middle of the shear span, in which the multiple fine cracks in the shear direction are increased and play a dominant role in increasing the shear stress from 3.3 MPa to 6.7 MPa (point 2 to 4). Thereafter, the localization of some multiple fine cracks occurs until the peak load with a 7.0 MPa shear stress (point 5), and the specimen is fractured in shear failure, in which the shear angle of the multiple fine cracks between the diagonal shear direction and the axial direction is about 28°. Figure 4 shows the fracture surface of the SHCC member in shear failure. It can be seen that the area near the loading point (yellow line area) is collapsed and the rest of the area (blue line area) is very smooth, whereas the fracture surface of normal concrete is quite rough. Hence, the fracture surface of the SHCC member is different from that of normal concrete. In order to understand the shear failure of SHCC, the fracture surface of the shear-failed SHCC member is surveyed. The shear stress is defined by the shear force divided by the effective section area of the effective height multiplied by width, and the maximum shear stresses of cases 2-100 and 3-100 are 9.8 MPa and 7.0 MPa, respectively, which means the specimen with a/d = 2 has a larger load carrying capacity than that of the specimens with a/d = 3. The shear stress-displacement curve also shows that the linear curve is near the maximum shear strength and the sudden shear failure occurs thereafter. This behavior may be due to the smooth crack surface of the SHCC member.

Characteristics of the SHCC Fracture Surface
The crack propagation pattern of the tested specimens corresponding to the points 1 to 5 in the shear stress-displacement curve is also demonstrated in Figure 3. In the case 2-100, the flexural multi-fine cracks first occur at point 1 (lower load level with a 2 MPa shear stress), and some multiple fine cracks occur in the middle of the shear span independent of the aforementioned flexural cracks at point 2 (load level with 4 MPa shear stress). At points 3 and 4 (load level with 7.3 MPa and 9.3 MPa shear stresses), the multiple fine cracks are gradually increased with the increasing load level, in which the multiple fine cracks in the shear direction play a dominant role in the SHCC member in shear failure. Moreover, it is obvious that the localization behavior occurs among the multiple fine cracks at point 5 (peak load with a 9.8 MPa shear stress), and the shear load suddenly drops thereafter, in which the shear angle of the multiple fine cracks between the diagonal shear direction and axial direction is about 33.5 • .
In the crack propagation pattern of case 3-100, it can be easily seen that this specimen shows similar crack propagation behavior to that of case 2-100. The flexural multi-fine cracks first occur at point 1 (a lower load level with a 1.7 MPa shear stress), then the multiple fine cracks occur in the middle of the shear span, in which the multiple fine cracks in the shear direction are increased and play a dominant role in increasing the shear stress from 3.3 MPa to 6.7 MPa (point 2 to 4). Thereafter, the localization of some multiple fine cracks occurs until the peak load with a 7.0 MPa shear stress (point 5), and the specimen is fractured in shear failure, in which the shear angle of the multiple fine cracks between the diagonal shear direction and the axial direction is about 28 • . Figure 4 shows the fracture surface of the SHCC member in shear failure. It can be seen that the area near the loading point (yellow line area) is collapsed and the rest of the area (blue line area) is very smooth, whereas the fracture surface of normal concrete is quite rough. Hence, the fracture surface of the SHCC member is different from that of normal concrete. In order to understand the shear failure of SHCC, the fracture surface of the shear-failed SHCC member is surveyed.  Figure 5 demonstrates the investigated roughness of the fracture surface in the shearfailed SHCC member, in which the sample is obtained from the coarse area near the loading point of the tested specimens. The sample length in the longitudinal direction is 125 mm, and the measured length is 123 mm, since the starting position for measurement has a distance of 2 mm from the side. The sample surface is measured using a laser displacement meter, which is implemented along the six dotted red lines, as marked in Figure 5, and six series of roughness (line 1 to line 6) along the dotted red lines are obtained.  Figure 5 demonstrates the investigated roughness of the fracture surface in the shearfailed SHCC member, in which the sample is obtained from the coarse area near the loading point of the tested specimens. The sample length in the longitudinal direction is 125 mm, and the measured length is 123 mm, since the starting position for measurement has a distance of 2 mm from the side. The sample surface is measured using a laser displacement meter, which is implemented along the six dotted red lines, as marked in Figure 5, and six series of roughness (line 1 to line 6) along the dotted red lines are obtained.  Figure 5 shows the revised test data of the measured surface roughness, in which the start position (0 mm position) in the fine area and the end position (125 mm position) in the coarse area have the same roughness change of 0 mm. It can be easily seen that the roughness change of the measured surface is serrated, with many peak-down phenomena, and the roughness in the coarse area is obviously larger and steeper than that in the fine area.  Figure 5 shows the revised test data of the measured surface roughness, in which the start position (0 mm position) in the fine area and the end position (125 mm position) in the coarse area have the same roughness change of 0 mm. It can be easily seen that the roughness change of the measured surface is serrated, with many peak-down phenomena, and the roughness in the coarse area is obviously larger and steeper than that in the fine area. Figure 6 shows the load-displacement curves of the SHCC layer strengthening the RC member in flexural failure, in which the tested specimens are loaded under a four-point bending setup. In the experiment, the RC beam is the control beam, and the cases SHCC-10, SHCC-30, and SHCC-50 represent RC beams strengthened by the SHCC layer with different thicknesses of 10 mm, 30 mm, and 50 mm, respectively [25]. The load is measured by a load cell, and the displacements at loading points and supporting points, as well as a middle point, are measured by displacement transducers, which are marked as A and B, respectively.

Load-Carrying Capacities of Strengthened RC Member in Flexural Failure
As shown in Figure 6, it can be clearly seen that the initial stiffness of the load-displacement curve of the strengthened RC member is increased with the increasing thickness of the SHCC layer. The load carrying capacity of the control beam is 40.8 kN, and those of the cases SHCC-10, SHCC-30, and SHCC-50 are 48.9 kN, 55 kN, and 65.7 kN, respectively, which increase by 16.5%, 34.8%, and 61.0% from that of the control beam. Moreover, the cases SHCC-10, SHCC-30, and SHCC-50 exhibit similar characteristics of load-carrying capacity. It is obvious that the initial stiffness of the strengthened RC member first changes after the yielding of the longitudinal reinforcements, and then the SHCC layer continues to carry the load with ductile behavior due to the multiple fine cracks in the SHCC layer. Thereafter, the load drops due to the lost load-carrying capacity of the SHCC layer, which is induced by localized multiple fine cracks in the SHCC layer, and then the load continues to drop due to the gradual failure in the RC substrate beam.

Crack Propagation Pattern of the SHCC Layer for Flexural Strengthening
The crack propagation pattern of the SHCC layer strengthening the RC member in flexural failure is demonstrated in Figure 7, which corresponds to the points 1 to 3 in the load-displacement curves in Figure 6. In the case of SHCC-10, two localized cracks first occur in the RC substrate beam without multiple fine cracks in the SHCC layer (point 1). After that, the zero-span tensile behavior is dominant in the SHCC layer adjacent to every localized crack in the RC substrate beam (point 2), in which multiple fine cracks occur due to the damage in the SHCC layer caused by the stress concentration from the localized crack in the RC beam. Specifically, those multiple fine cracks from the zero-span tensile behavior are around the localized crack in the RC beam, which spread from the top to the

Load-Carrying Capacities of Strengthened RC Member in Flexural Failure
As shown in Figure 6, it can be clearly seen that the initial stiffness of the loaddisplacement curve of the strengthened RC member is increased with the increasing thickness of the SHCC layer. The load carrying capacity of the control beam is 40.8 kN, and those of the cases SHCC-10, SHCC-30, and SHCC-50 are 48.9 kN, 55 kN, and 65.7 kN, respectively, which increase by 16.5%, 34.8%, and 61.0% from that of the control beam. Moreover, the cases SHCC-10, SHCC-30, and SHCC-50 exhibit similar characteristics of load-carrying capacity. It is obvious that the initial stiffness of the strengthened RC member first changes after the yielding of the longitudinal reinforcements, and then the SHCC layer continues to carry the load with ductile behavior due to the multiple fine cracks in the SHCC layer. Thereafter, the load drops due to the lost load-carrying capacity of the SHCC layer, which is induced by localized multiple fine cracks in the SHCC layer, and then the load continues to drop due to the gradual failure in the RC substrate beam.

Crack Propagation Pattern of the SHCC Layer for Flexural Strengthening
The crack propagation pattern of the SHCC layer strengthening the RC member in flexural failure is demonstrated in Figure 7, which corresponds to the points 1 to 3 in the load-displacement curves in Figure 6. In the case of SHCC-10, two localized cracks first occur in the RC substrate beam without multiple fine cracks in the SHCC layer (point 1). After that, the zero-span tensile behavior is dominant in the SHCC layer adjacent to every localized crack in the RC substrate beam (point 2), in which multiple fine cracks occur due to the damage in the SHCC layer caused by the stress concentration from the localized crack in the RC beam. Specifically, those multiple fine cracks from the zero-span tensile behavior are around the localized crack in the RC beam, which spread from the top to the bottom of the SHCC layer. Thereafter, some multiple fine cracks localize in the SHCC layer, which gradually lessens the load-carrying capacity (point 3), and the debonding behavior occurs between the SHCC layer and the RC beam. In the case of SHCC-30, two types of multiple fine cracks first distribute in the SHCC layer (point 1). One type comes from the aforementioned zero-span tensile behavior with many multiple fine cracks around the localized crack in the RC beam, and another type comes from the uniaxial tensile behavior, with many multiple fine cracks spread from the bottom to the top of the SHCC layer. Specifically, those multiple fine cracks from the uniaxial tensile behavior occur due to the bending effect of the strengthened RC beam. Moreover, the SHCC strengthening layer loses its load-carrying capacity due to completing the localization of the multiple fine cracks thereafter, in which the SHCC layer cannot constrain the localized crack in the RC beam to open, and debonding behavior occurs at last. In the case of SHCC-50, the multiple fine cracks in the SHCC layer are almost evenly distributed from the aforementioned uniaxial tensile behavior (point 1), which is increased and quite different from cases SHCC-10 and SHCC-30 (point 2). This may imply that the uniaxial tensile behavior comes from the bending effect that plays a dominant role in the crack propagation pattern in the case of SHCC-50, in which the multiple fine cracks spread from the bottom to the top of the SHCC layer.
Materials 2023, 16, x FOR PEER REVIEW 9 of 13 SHCC layer. Specifically, those multiple fine cracks from the uniaxial tensile behavior occur due to the bending effect of the strengthened RC beam. Moreover, the SHCC strengthening layer loses its load-carrying capacity due to completing the localization of the multiple fine cracks thereafter, in which the SHCC layer cannot constrain the localized crack in the RC beam to open, and debonding behavior occurs at last. In the case of SHCC-50, the multiple fine cracks in the SHCC layer are almost evenly distributed from the aforementioned uniaxial tensile behavior (point 1), which is increased and quite different from cases SHCC-10 and SHCC-30 (point 2). This may imply that the uniaxial tensile behavior comes from the bending effect that plays a dominant role in the crack propagation pattern in the case of SHCC-50, in which the multiple fine cracks spread from the bottom to the top of the SHCC layer.  Figure 8 demonstrates the crack patterns of the SHCC layer used for the flexural strengthening of the RC member with varied thicknesses of the SHCC layer, compared with the crack pattern of SHCC in uniaxial tension, as shown in the aforementioned Figure  1. It can be clearly seen that the multiple fine cracks are evenly distributed in the uniaxial tensile behavior, whereas those of the SHCC strengthening layer are differently distributed. On one hand, the multiple fine cracks in the SHCC strengthening layer are limited, adjacent to the localized crack in the RC member, while the thickness of the SHCC strengthening layer is small. On the other hand, most of the multiple fine cracks are evenly distributed, while the thickness of the SHCC layer is large, except for the concentrated multiple fine cracks around the localized crack in the RC member. As a result, the obvious advantage of the high ductility of the SHCC material in uniaxial tension is reduced by the effect of the existing crack within the RC substrate member in repair or strengthening applications. Moreover, the zero-span tensile test is recommended to assess the crack opening performance of the thin-thickness SHCC layer used for RC flexural strengthening, in which the crack distribution within the SHCC layer is limited near the existing crack in the RC structure [25][26][27]. As shown in Figure 8, the zero-span tensile test is implemented using the zero span formed by a pair of steel plates, which is modeled consid-   Figure 1. It can be clearly seen that the multiple fine cracks are evenly distributed in the uniaxial tensile behavior, whereas those of the SHCC strengthening layer are differently distributed. On one hand, the multiple fine cracks in the SHCC strengthening layer are limited, adjacent to the localized crack in the RC member, while the thickness of the SHCC strengthening layer is small. On the other hand, most of the multiple fine cracks are evenly distributed, while the thickness of the SHCC layer is large, except for the concentrated multiple fine cracks around the localized crack in the RC member. As a result, the obvious advantage of the high ductility of the SHCC material in uniaxial tension is reduced by the effect of the existing crack within the RC substrate member in repair or strengthening applications. Moreover, the zero-span tensile test is recommended to assess the crack opening performance of the thin-thickness SHCC layer used for RC flexural strengthening, in which the crack distribution within the SHCC layer is limited near the existing crack in the RC structure [25][26][27]. As shown in Figure 8, the zero-span tensile test is implemented using the zero span formed by a pair of steel plates, which is modeled considering the condition near the crack in the RC substrate member.  Figure 9 shows the load-displacement curves and the crack distributions of the RC member with shear strengthening using the SHCC layer, in which the ratios of the shear span length to the effective depth of all the specimens are 3. The RC beam is the control beam [28], and the case SHCC-3 represents the RC beam strengthened by a 3-mm thick SHCC layer on both sides. Both the RC beam and SHCC-3 have a 1200-mm length and a 100 mm × 200 mm cross-section, in which there are no web reinforcements, and two 10mm-diameter deformed bars are arranged along the longitudinal direction. In the case of SHCC-3, both side surfaces of the RC beams are roughed by washing out using a retarder, which are adopted as the interfaces of the RC member and the SHCC strengthening layers; the SHCC layers are cast at both sides of the RC beam thereafter. In the experiment, the specimens are loaded under a four-point bending setup; the displacements at the loading points and the support point, as well as the middle point, are measured by displacement transducers, and the load is measured by a load cell, which are marked as A and B, respectively.  Figure 9 shows the load-displacement curves and the crack distributions of the RC member with shear strengthening using the SHCC layer, in which the ratios of the shear span length to the effective depth of all the specimens are 3. The RC beam is the control beam [28], and the case SHCC-3 represents the RC beam strengthened by a 3-mm thick SHCC layer on both sides. Both the RC beam and SHCC-3 have a 1200-mm length and a 100 mm × 200 mm cross-section, in which there are no web reinforcements, and two 10-mm-diameter deformed bars are arranged along the longitudinal direction. In the case of SHCC-3, both side surfaces of the RC beams are roughed by washing out using a retarder, which are adopted as the interfaces of the RC member and the SHCC strengthening layers; the SHCC layers are cast at both sides of the RC beam thereafter. In the experiment, the specimens are loaded under a four-point bending setup; the displacements at the loading points and the support point, as well as the middle point, are measured by displacement transducers, and the load is measured by a load cell, which are marked as A and B, respectively.

Behavior of the RC Member with Shear Strengthening Using the SHCC Layer
As shown in Figure 9, it can be clearly seen that the shear load-carrying capacity of the shear-strengthened RC member is significantly increased, even with the small thickness of the SHCC strengthening layer. This excellent strengthening effect probably originates from the shear stress transfer behavior on the localized crack surface of the SHCC strengthening layer, which is contributed to by both the contact stress and fiber bridging stress on the crack surface of the SHCC layer.
Moreover, it can be seen that the observed multiple fine cracks in the diagonal shear direction of the SHCC strengthening layer are obviously decreased from those of the aforementioned shear-failed SHCC member, as demonstrated in Figure 3. This implies that the aforementioned zero-span tensile behavior is also observed in the SHCC layer used for shear strengthening the RC member, in which the crack distribution within the SHCC layer is limited near the existing crack in the RC member. Therefore, the ductility of SHCC is also reduced while it is used for the shear-strengthening RC member, which is similar to that of SHCC used for the flexural-strengthening RC member. As shown in Figure 9, it can be clearly seen that the shear load-carrying capacity of the shear-strengthened RC member is significantly increased, even with the small thickness of the SHCC strengthening layer. This excellent strengthening effect probably originates from the shear stress transfer behavior on the localized crack surface of the SHCC strengthening layer, which is contributed to by both the contact stress and fiber bridging stress on the crack surface of the SHCC layer.
Moreover, it can be seen that the observed multiple fine cracks in the diagonal shear direction of the SHCC strengthening layer are obviously decreased from those of the aforementioned shear-failed SHCC member, as demonstrated in Figure 3. This implies that the aforementioned zero-span tensile behavior is also observed in the SHCC layer used for shear strengthening the RC member, in which the crack distribution within the SHCC layer is limited near the existing crack in the RC member. Therefore, the ductility of SHCC is also reduced while it is used for the shear-strengthening RC member, which is similar to that of SHCC used for the flexural-strengthening RC member.

Conclusions
In this paper, the structural performance of SHCC is reviewed, focusing on the flexural and shear behavior of the SHCC member and the SHCC layer used for strengthening the RC member; the following conclusions can be drawn.
(1) The shear-failed SHCC members are suddenly dropped and smoothly decreased after reaching the peak load capacity, in which the shear-failed SHCC members demonstrate similar crack propagation behavior. The flexural multi-fine cracks first occur, and then some multiple fine cracks occur in the middle of the shear span independent of the aforementioned flexural cracks, in which the multiple fine cracks are gradually increased with the increasing load level thereafter. The multiple fine cracks in the shear direction play a dominant role in the SHCC member during shear failure. After that, the localization behavior occurs among the multiple fine cracks and the sudden drop in shear load appears in the shear-failed SHCC member. (2) The crack propagation patterns of the SHCC member in flexural failure demonstrate that the flexural multiple fine cracks appear in the SHCC member first, and then the multiple fine cracks significantly increase with the increasing load-carrying capacity.

Conclusions
In this paper, the structural performance of SHCC is reviewed, focusing on the flexural and shear behavior of the SHCC member and the SHCC layer used for strengthening the RC member; the following conclusions can be drawn.
(1) The shear-failed SHCC members are suddenly dropped and smoothly decreased after reaching the peak load capacity, in which the shear-failed SHCC members demonstrate similar crack propagation behavior. The flexural multi-fine cracks first occur, and then some multiple fine cracks occur in the middle of the shear span independent of the aforementioned flexural cracks, in which the multiple fine cracks are gradually increased with the increasing load level thereafter. The multiple fine cracks in the shear direction play a dominant role in the SHCC member during shear failure. After that, the localization behavior occurs among the multiple fine cracks and the sudden drop in shear load appears in the shear-failed SHCC member. (2) The crack propagation patterns of the SHCC member in flexural failure demonstrate that the flexural multiple fine cracks appear in the SHCC member first, and then the multiple fine cracks significantly increase with the increasing load-carrying capacity. This means that the SHCC member experiences multi-cracking processes, which finally fail due to the localization of some flexural multiple fine cracks in the SHCC member thereafter. (3) In the load-displacement curves of the SHCC layer flexural strengthening the RC member, the initial stiffness of the strengthened RC member first changes after yielding the longitudinal reinforcements, and then the SHCC layer carries the load with ductile behavior due to the multiple fine cracks in the SHCC layer. Thereafter, the load drops due to the decreased load-carrying capacity of the SHCC layer, which is induced by the localized multiple fine cracks in the SHCC layer, and then the load continues to drop due to the gradual failure in the RC substrate beam. There are two types of crack propagation patterns in the multiple fine cracks in the SHCC layer used for the flexural strengthening of the RC member, which is influenced by the varied thicknesses of the SHCC layer. The multiple fine cracks from the zero-span tensile behavior are around the localized crack in the RC beam and spread from the top to the bottom of the SHCC layer, which are caused by the stress concentration from the localized crack in the RC beam. The multiple fine cracks from the uniaxial tensile behavior are almost evenly distributed and spread from the bottom to the top of the SHCC layer, which is induced by the bending effect of the strengthened RC beam. (4) In the SHCC layer used for the shear strengthening of the RC member, the SHCC layer can significantly increase the shear load-carrying capacity of the strengthened RC member. This excellent strengthening effect is probably due to the shear stress transfer behavior on the localized crack surface of the SHCC strengthening layer, which is contributed to by both the contact stress and fiber bridging stress on the crack surface of the SHCC layer. Moreover, the multiple fine cracks in the diagonal shear direction of the SHCC strengthening layer are obviously decreased from those of the shear-failed SHCC member, which means that the ductility of the SHCC layer is reduced while it is used for shear strengthening. Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest:
The authors declare that there are no conflict of interest regarding the publication of this paper.